I don't think it's fair to say we "want to lock people up". We need to believe that we can make ourselves safe from criminals. We need to believe can do some science magic and tell the difference between the criminals and the regular people so we can sleep at night. Admitting we can't calls in to question the whole premise of locking the bad people up. A judge or politician can't come out and say, "we have no reliable way of deciding guilt or innocence." That is reality but we can't allow ourselves to admit it.